I've since been re-reading Christopher Andrew's official history of MI5, as part of my research. Not a promising source on collusion, you might think but while Andrew covers relatively little ground in relation to MI5 in Ireland before the Troubles, he does include one particularly damning piece of evidence from the very earliest history of the organisation.
It concerns the interrogation of Sir Roger Casement, following his landing in Ireland from Germany ahead of the Easter Rising in 1916:
After his arrest, Casement was jointly interrogated at Scotland Yard by Thomson, Captain Reginald 'Blinker' Hall, the DNI, and MI5's main Irish expert, the Old Harrovian Major (later Lieutenant Colonel) Frank Hall (no relation to the DNI), a landowner from County Down. The many conspiracy theorists attracted by the Casement affair have surprisingly failed to notice that before the war Frank Hall had been military secretary of the Ulster Volunteer Force and a gun-runner himself .
Andrew's source for this is Jeff Dudgeon whose earlier work on Casement reports that Hall was offered 'a job in Military Intelligence by a top Army Unionist', following the incorporation of the UVF into the army in 1914 .
This is a particularly suggestive comment in the light of the Curragh Mutiny earlier that year, when the Army had effectively backed the UVF in facing down the Home Rule policy of the Liberal Government.
Not to put too fine a point on it, there's a fairly sound case that MI5 was under the influence of right-wing subversives from the beginning.
Postscript: As it happens, one of my patrons, Pete Middleton, is running a Roger Casement and Irish in Islington Historical Walking Tour this Saturday.
 Andrew, Christopher; Defence of the Realm, The Authorized History of MI5, Allen Lane, 2009, p.87.
 Jeff Dudgeon, UVF and MI5: the double life of Q, Belfast Telegraph, archived at Ulster-Scots and Irish Unionist Resource, accessed 1 August 2017.